MODULE 1

Electrical Drives

Electrical Drives:

Motion control is required in large number of industrial and domestic applications like

transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots,

washing machines etc.

Systems employed for motion control are called DRIVES, and may employ any of prime

movers such as diesel or petrol engines, gas or steam turbines, steam engines, hydraulic motors and

electric motors, for supplying mechanical energy for motion control. Drives employing electric motors

are known as ELECTRICAL DRIVES.

An ELECTRIC DRIVE can be defined as an electromechanical device for converting electrical

energy into mechanical energy to impart motion to different machines and mechanisms for various

kinds of process control.

Classification of Electric Drives

According to Mode of Operation

Continuous duty drives

Short time duty drives

Intermittent duty drives

According to Means of Control

Manual

Semi automatic

Automatic

Individual drive

Group drive

Multi-motor drive

According to Dynamics and Transients

Uncontrolled transient period

Controlled transient period

According to Methods of Speed Control

Reversible and non-reversible uncontrolled constant speed.

Reversible and non-reversible step speed control.

Variable position control.

Reversible and non-reversible smooth speed control.

1. They have flexible control characteristics. The steady state and dynamic characteristics of

electric drives can be shaped to satisfy the load requirements.

2. Drives can be provided with automatic fault detection systems. Programmable logic controller

and computers can be employed to automatically control the drive operations in a desired

sequence.

3. They are available in wide range of torque, speed and power.

4. They are adaptable to almost any operating conditions such as explosive and radioactive

environments

5. It can operate in all the four quadrants of speed-torque plane

6. They can be started instantly and can immediately be fully loaded

7. Control gear requirement for speed control, starting and braking is usually simple and easy to

operate.

Choice of an electric drive depends on a number of factors. Some of the important factors

are. 1. Steady State Operating conditions requirements

Nature of speed torque characteristics, speed regulation, speed range, efficiency, duty

cycle, quadrants of operation, speed fluctuations if any, ratings etc

2. Transient operation requirements

Values of acceleration and deceleration, starting, braking and reversing

performance. 3. Requirements related to the source

Types of source and its capacity, magnitude of voltage, voltage fluctuations, power

factor, harmonics and their effect on other loads, ability to accept regenerative power

4. Capital and running cost, maintenance needs life.

5. Space and weight restriction if any.

6. Environment and location.

7. Reliability.

Group Electric Drive

This drive consists of a single motor, which drives one or more line shafts supported on

bearings. The line shaft may be fitted with either pulleys and belts or gears, by means of which a group

of machines or mechanisms may be operated. It is also some times called as SHAFT DRIVES. Advantages

A single large motor can be used instead of number of small motors

Disadvantages

There is no flexibility. If the single motor used develops fault, the whole process will be

stopped.

Individual Electric Drive

In this drive each individual machine is driven by a separate motor. This motor also imparts

motion to various parts of the machine. Multi Motor Electric Drive

In this drive system, there are several drives, each of which serves to actuate one of the

working parts of the drive mechanisms.

E.g.: Complicated metal cutting machine tools

Paper making industries,

Rolling machines etc.

General Electric Drive System

Block diagram of an electric drive system is shown in the figure below.

A modern variable speed electrical drive system has the following components

Electrical machines and loads

Power Modulator

Sources

Control unit

Sensing unit

Electrical Machines

Most commonly used electrical machines for speed control applications are the following

DC Machines

Shunt, series, compound, separately excited DC motors and switched reluctance machines.

AC Machines

Induction, wound rotor, synchronous, PM synchronous and synchronous reluctance

machines. Special Machines

Brush less DC motors, stepper motors, switched reluctance motors are used.

Power Modulators

Functions:

Modulates flow of power from the source to the motor in such a manner that

motor is imparted speed-torque characteristics required by the load

During transient operation, such as starting, braking and speed reversal, it

restricts source and motor currents with in permissible limits.

It converts electrical energy of the source in the form of suitable to the motor

Selects the mode of operation of the motor (i.e.) Motoring and Braking.

In the electric drive system, the power modulators can be any one of the following

Controlled rectifiers (ac to dc converters)

Inverters (dc to ac converters)

AC voltage controllers (AC to AC converters)

DC choppers (DC to DC converters)

Cyclo converters (Frequency conversion)

Electrical Sources

Very low power drives are generally fed from single phase sources. Rest of the drives is

powered from a 3 phase source. Low and medium power motors are fed from a 400v supply. For

higher ratings, motors may be rated at 3.3KV, 6.6KV and 11 KV. Some drives are powered from

battery.

Sensing Unit

Speed Sensing (From Motor)

Torque Sensing

Position Sensing

Current sensing and Voltage Sensing from Lines or from motor terminals

From Load

Torque sensing

Temperature Sensing

Control Unit

Control unit for a power modulator are provided in the control unit. It matches the motor and

power converter to meet the load requirements.

Classification of Electrical Drives

Another main classification of electric drive is

DC drive

AC drive

Comparison between DC and AC drives

DC DRIVES AC DRIVES

The power circuit and control circuit The power circuit and control circuit are

is simple and inexpensive complex

It requires frequent maintenance Less Maintenance

The commutator makes the motor These problems are not there in these motors

bulky, costly and heavy and are inexpensive, particularly squirrel cage

induction motors

Fast response and wide speed range In solid state control the speed range is wide

of control, can be achieved smoothly and conventional method is stepped and

by conventional and solid state limited

control

Speed and design ratings are limited Speed and design ratings have upper limits

due to commutations

Applications

Paper mills

Cement Mills

Textile mills

Sugar Mills

Steel Mills

Electric Traction

Petrochemical Industries

Electrical Vehicles

Dynamics of Motor Load System

Fundamentals of Torque Equations

A motor generally drives a load (Machines) through some transmission system. While

motor always rotates, the load may rotate or undergo a translational motion.

Load speed may be different from that of motor, and if the load has many parts, their

speed may be different and while some parts rotate others may go through a translational motion.

Equivalent rotational system of motor and load is shown in the figure.

T

ωm

Tl

Motor

Load

Notations Used:

J = Moment of inertia of motor load system referred to the motor shaft kg − m2

ωm = Instantaneous angular velocity of motor shaft, rad/sec. T =

Instantaneous value of developed motor torque, N-m

Tl = Instantaneous value of load torque, referred to the motor shaft N-m

Load torque includes friction and wind age torque of motor. Motor-load system shown in

figure can be described by the following fundamental torque equation.

T − Tl = d

(Jωm ) = J dω

m + ωm

dJ − − − − − − − − − − − (1)

dtdtdt

Equation (1) is applicable to variable inertia drives such as mine winders, reel drives, Industrial robots.

For drives with constant inertia dJ

= 0

dt

∴T = Tl + J dω

m − − − − − − − − − − −

−(2) dt

Equation (2) shows that torque developed by motor is counter balanced by load torque Tl and a

dω m

dω m

dynamic torque J

. Torque component J is called dynamic torque because it is present

dt dt

only during the transient operations.

Note:

Energy associated with dynamic torque

by

Jωm

2 .

2

Classification of Load Torques:

dω m

J is stored in the form of kinetic energy given

dt

Various load torques can be classified into broad categories.

Active load torques

Passive load torques

Load torques which has the potential to drive the motor under equilibrium conditions are called

active load torques. Such load torques usually retain their sign when the drive rotation is changed

(reversed)

Eg: Torque due to force of gravity

Torque due tension

Torque due to compression and torsion etc

Load torques which always oppose the motion and change their sign on the reversal of motion

are called passive load torques

Eg: Torque due to friction, cutting etc.

Components of Load Torques:

The load torque Tl can be further divided in to following components

(i) Friction Torque (TF)

Friction will be present at the motor shaft and also in various parts of the

load. TF is the equivalent value of various friction torques referred to the

motor shaft.

(ii) Windage Torque (TW)

When motor runs, wind generates a torque opposing the motion. This is

known as windage torque.

(iii) Torque required to do useful mechanical work.

Nature of this torque depends upon particular application. It may be

constant and independent of speed. It may be some function of speed, it

may be time invariant or time variant, its nature may also change with

the load’s mode of operation.

Value of friction torque with speed is shown in figure below

ωm

TF

Its value at stand still is much higher than its value slightly above zero speed. Friction at zero speed is

called stiction or static friction. In order to start the drive the motor should at least exceed stiction.

Friction torque can also be resolved into three components

Tv Speed Tc

Ts

Torque

Component Tv varies linearly with speed is called VISCOUS friction and is given by

Tv Bωm

Where B is viscous friction co-efficient.

Another component TC, which is independent of speed, is known as COULOMB friction. Third

component Ts accounts for additional torque present at stand still. Since Ts is present only at stand still

it is not taken into account in the dynamic analysis. Windage torque, TW which is proportional to

speed squared is given by

Tw Cωm

2

C is a constant

From the above discussions, for finite speed

Tl TL Bωm TC Cωm 2

Characteristics of Different types of Loads

One of the essential requirements in the section of a particular type of motor for driving a

machine is the matching of speed-torque characteristics of the given drive unit and that of the motor.

Therefore the knowledge of how the load torque varies with speed of the driven machine is necessary.

Different types of loads exhibit different speed torque characteristics. However, most of the industrial

loads can be classified into the following four categories.

Constant torque type load

Torque proportional to speed (Generator Type load)

Torque proportional to square of the speed (Fan type load)

Torque inversely proportional to speed (Constant power type load)

Constant Torque characteristics:

Most of the working machines that have mechanical nature of work like shaping, cutting,

grinding or shearing, require constant torque irrespective of speed. Similarly cranes during the hoisting

and conveyors handling constant weight of material per unit time also exhibit this type of

characteristics.

Speed

TL

T=K

Torque

Torque Proportional to speed:

Separately excited dc generators connected to a constant resistance load, eddy current

brakes have speed torque characteristics given by T=kω

Speed TL

Torque

Torque proportional to square of the speed:

Another type of load met in practice is the one in which load torque is proportional to the

square of the speed.Eg Fans rotary pumps, compressors and ship propellers.

Speed

TL

T Kω 2

Torque

Torque Inversely proportional to speed:

Certain types of lathes, boring machines, milling machines, steel mill coiler and electric

traction load exhibit hyperbolic speed-torque characteristics.

Speed TL

Tα ω1

Torque

Multi quadrant Operation:

For consideration of multi quadrant operation of drives, it is useful to establish suitable

conventions about the signs of torque and speed. A motor operates in two modes – Motoring and

braking. In motoring, it converts electrical energy into mechanical energy, which supports its motion.

In braking it works as a generator converting mechanical energy into electrical energy and thus

opposes the motion. Motor can provide motoring and braking operations for both forward and reverse

directions. Figure shows the torque and speed co-ordinates for both forward and reverse motions.

Power developed by a motor is given by the product of speed and torque. For motoring operations

power developed is positive and for braking operations power developed is negative.

Speed

Forward

Braking

Forward Motoring

II

I

Torque

III IV

Reverse Reverse

Motoring Braking

In quadrant I, developed power is positive, hence machine works as a motor supplying

mechanical energy. Operation in quadrant I is therefore called Forward Motoring. In quadrant II,

power developed is negative. Hence, machine works under braking opposing the motion. Therefore

operation in quadrant II is known as forward braking. Similarly operation in quadrant III and IV can

be identified as reverse motoring and reverse braking since speed in these quadrants is negative. For

better understanding of the above notations, let us consider operation of hoist in four quadrants as

shown in the figure. Direction of motor and load torques and direction of speed are marked by arrows.

T Tl

ωm

Motion

T

Tl

ωm

Motion

Counter weight

T

Empty Cage

Counter

II I weight Loaded

Cage

III IV T

Tl

ωm

Tl ωm

Counter weight

Motion

Empty Cage

Load Torque with empty cage

Motion

Loaded Cage

Counter weight

Load Torque with loaded cage

A hoist consists of a rope wound on a drum coupled to the motor shaft one end of the rope is

tied to a cage which is used to transport man or material from one level to another level . Other end of

the rope has a counter weight. Weight of the counter weight is chosen to be higher than the weight of

empty cage but lower than of a fully loaded cage. Forward direction of motor speed will be one which

gives upward motion of the cage. Load torque line in quadrants I and IV represents speed-torque

characteristics of the loaded hoist. This torque is the difference of torques due to loaded hoist and counter

weight.

The load torque in quadrants II and III is the speed torque characteristics for an empty hoist.

This torque is the difference of torques due to counter weight and the empty hoist. Its sigh is negative

because the counter weight is always higher than that of an empty cage.

The quadrant I operation of a hoist requires movement of cage upward, which corresponds to

the positive motor speed which is in counter clockwise direction here. This motion will be obtained if

the motor products positive torque in CCW direction equal to the magnitude of load torque TL1. Since

developed power is positive, this is forward motoring operation. Quadrant IV is obtained when a

loaded cage is lowered. Since the weight of the loaded cage is higher than that of the counter weight .It

is able to overcome due to gravity itself.

In order to limit the cage within a safe value, motor must produce a positive torque T equal to

TL2 in anticlockwise direction. As both power and speed are negative, drive is operating in reverse

braking operation. Operation in quadrant II is obtained when an empty cage is moved up. Since a

counter weigh is heavier than an empty cage, its able to pull it up. In order to limit the speed within a

safe value, motor must produce a braking torque equal to TL2 in clockwise direction. Since speed is

positive and developed power is negative, it’s forward braking operation.

Operation in quadrant III is obtained when an empty cage is lowered. Since an empty cage has

a lesser weight than a counter weight, the motor should produce a torque in CW direction. Since speed

is negative and developed power is positive, this is reverse motoring operation. Steady State Stability:

Equilibrium speed of motor-load system can be obtained when motor torque equals the load

torque. Electric drive system will operate in steady state at this speed, provided it is the speed of stable

state equilibrium. Concept of steady state stability has been developed to readily evaluate the stability

of an equilibrium point from the steady state speed torque curves of the motor and load system.

In most of the electrical drives, the electrical time constant of the motor is negligible compared

with the mechanical time constant. During transient condition, electrical motor can be assumed to be

in electrical equilibrium implying that steady state speed torque curves are also applicable to the

transient state operation.

Now, consider the steady state equilibrium point A shown in figure below

ωm T

TL

A

ωm

Tshift TA TM Torque

The equilibrium point will be termed as stable state when the operation will be restored to it

after a small departure from it due to disturbance in the motor or load. Due to disturbance a reduction

of ωm in speed at new speed, electrical motor torque is greater than the load torque, consequently

motor will accelerate and operation will be restores to point A. similarly an increase in ωm speed

caused by a disturbance will make load torque greater than the motor torque, resulting into

deceleration and restoring of operation to point A.

Now consider equilibrium point B which is obtained when the same motor drives another load

as shown in the figure. A decrease in speed causes the load torque to become greater than the motor

torque, electric drive decelerates and operating point moves away from point B. Similarly when

working at point B and increase in speed will make motor torque greater than the load torque, which

will move the operating point away from point B

ωm T

B

ωm

TL

Tshift T

A TM Torque

From the above discussions, an equilibrium point will be stable when an increase in speed causes load-

torque to exceed the motor torque. (i.e.) When at equilibrium point following conditions is satisfied.

dTL > dT − − − − − − − − − − − − − (1)

dωm dωm

Inequality in the above equation can be derived by an alternative approach. Let a small perturbation in

speed, ωm results in T and Tl perturbation in T and Tl respectively. Therefore the general load-torque

equation becomes

T +

= T +

The general equation is

T = Tl + Tl + Jdωm + ωm

dt

T = T + T + Jdωm + J d ωm − − − −(2)

l l dt dt

T = Tl + J dω

m − − − − − − − (3) dt

Subtracting (3) from (2) and rearranging

J dω

m = T − T − − − − − − − − − (4)

dt l

From small perturbations, the speed –torque curves of the motor and load can be assumed to be

straight lines, thus

dT

T = ω

m − − − − − − − (5)

dω

m dT

T = l ω − − − − − − − (6)

m l

dω

m

Where dT and dTl are respectively slopes of the steady state speed torque curves of motor and

dωm dωm

load at operating point under considerations. Substituting (5) and (6) in (4) we get,

d ω m

dT dT J + l − ω = 0 − − − − − (7)

m

dt

dωm dωm

This is a first order linear differential equation. If initial deviation in speed at t=0 be ωm 0 then the

solution of equation (7) is

= ωm 0

1 dT dT

l

ωm exp −

−

− − − − − (8) t

J dω

m dω

m

An operating point will be stable when ωm approaches zero as t approaches infinity. For this to happen

exponential term in equation (8) should be negative.

Basics of Regenerative Braking

In the regenerative braking operation, the motor operates as generator, while it is still

connected to the supply. Here, the motor speed is greater than the synchronous speed. Mechanical

energy is converted into electrical energy, part of which is returned to the supply and rest of the energy

is last as heat in the winding and bearings of electrical machines pass smoothly from motoring region

to generating region, when over driven by the load.

An example of regenerative braking is shown in the figure below. Here an electric motor is

driving a trolley bus in the uphill and downhill direction. The gravity force can be resolved into two

components in the uphill direction. One is perpendicular to the load surface (F) and another one is

parallel to the road surface Fl. The parallel force pulls the motor towards bottom of the hill. If we

neglect the rotational losses, the motor must produce force Fm opposite to Fl to move the bus in the

uphill direction.

Fm

Fm

F

Fl Down Uphill F

Fl

Hill

This operation is indicated as shown in the figure below in the first quadrant. Here the power flow is

from the motor to load.

DOWN HILL

UPHILL Speed

Power Flow

Power Flow

TL

Speed TM

M LOAD M LOAD

Speed

TL

TM

Torque

Now we consider that the same bus is traveling in down hill, the gravitational force doesn’t change its

direction but the load torque pushes the motor towards the bottom of the hill. The motor produces a

torque in the reverse direction because the direction of the motor torque is always opposite to the

direction of the load torque. Here the motor is still in the same direction on both sides of the hill. This

is known as regenerative braking. The energy is exchange under regenerative braking operation is

power flows from mechanical load to source. Hence, the load is driving the machine and the machine

is generating electric power that is returned to the supply.

Regenerative braking of Induction motor:

An induction motor is subjected to regenerative braking, if the motor rotates in the same

direction as that of the stator magnetic field, but with a speed greater than the synchronous speed. Such

a state occurs during any one of the following process.

Downward motion of a loaded hoisting mechanism

During flux weakening mode of operation of IM.

Under regenerative braking mode, the machine acts as an induction generator. The induction

generator generates electric power and this power is fed back to the supply. This machine takes only

the reactive power for excitation. The speed torque characteristic of the motor for regenerative braking

is shown in the figure.

Speed Braking

Motoring

Torque

Regenerative Braking for DC motor:

In regenerative braking of dc motor, generated energy is supplied to the source. For this the following

condition is to be satisfied.

E > V and Ia should be negative

Speed

Motoring

Braking

Torque

Modes of Operation:

An electrical drive operates in three modes:

Steady state

Acceleration including Starting

Deceleration including Stopping dω

We know that T Tl J m

According to the above expression the steady state operation takes place when motor torque

equals the load torque. The steady state operation for a given speed is realized by adjustment of steady

state motor speed torque curve such that the motor and load torques are equal at this speed. Change in

speed is achieved by varying the steady state motor speed torque curve so that motor torque equals the

load torque at the new desired speed. In the figure shown below when the motor parameters are

adjusted to provide speed torque curve 1, drive runs at the desired speedωm1 . Speed is changed to ωm2

when the motor parameters are adjusted to provide speed torque curve 2. When load torque opposes

motion, the motor works as a motor operating in quadrant I or III depending on the direction of

rotation. When the load is active it can reverse its sign and act to assist the motion. Steady state

operation for such a case can be obtained by adding a mechanical brake which will produce a torque in

a direction to oppose the motion. The steady state operation is obtained at a speed for which braking

torque equal the load torque. Drive operates in quadrant II or IV depending upon the rotation.

Tl

ω m

ω m1 1

ωm 2

2

Torque

Acceleration and Deceleration modes are transient modes. Drive operates in acceleration mode

whenever an increase in its speed is required. For this motor speed torque curve must be changed so

that motor torque exceeds the load torque. Time taken for a given change in speed depends on inertia

of motor load system and the amount by which motor torque exceeds the load torque.

Increase in motor torque is accompanied by an increase in motor current. Care must be taken to

restrict the motor current with in a value which is safe for both motor and power modulator. In

applications involving acceleration periods of long duration, current must not be allowed to exceed the

rated value. When acceleration periods are of short duration a current higher than the rated value is

allowed during acceleration. In closed loop drives requiring fast response, motor current may be

intentionally forced to the maximum value in order to achieve high acceleration.

Figure shown below shows the transition from operating point A at speed ωm1 to operating

point B at a higher speedωm2 , when the motor torque is held constant during acceleration. The path

consists of AD1E1B. In the figure below, 1 to 5 are motor speed torque curves. Starting is a special

case of acceleration where a speed change from 0 to a desired speed takes place. All points mentioned

in relation to acceleration are applicable to starting. The maximum current allowed should not only be

safe for motor and power modulator but drop in source voltage caused due to it should also be in

acceptable limits. In some applications the motor should accelerate smoothly, without any jerk. This is

achieved when the starting torque can be increased step lessly from its zero value. Such a start is

known as soft start.

ω M

ωM 2

1

B E1

D3

ωM 1 2

Deceleration 3

A D1

ωM 3

4

E2 E3 5 C

-T T

Motor operation in deceleration mode is required when a decrease in its speed is required. According

to the equationT T J dω

m , deceleration occurs when load torque exceeds the motor torque. In

l dt

those applications where load torque is always present with substantial magnitude, enough

deceleration can be achieved by simply reducing the motor torque to zero. In those applications where

load torque may not always have substantial amount or where simply reducing the motor torque to

zero does not provide enough deceleration, mechanical brakes may be used to produce the required

magnitude of deceleration. Alternatively, electric braking may be employed. Now both motor and the

load torque oppose the motion, thus producing larger deceleration. During electric braking motor

current tends to exceed the safe limit. Appropriate changes are made to ensure that the current is

restricted within the safe limit.

Figure shown above shows paths followed during transition from point A at speed ωm1 to a

point C at a lower speedωm3 .When deceleration is carried out using electric braking at a constant

braking torque, the operating point moves along the path AD3E3C. When sufficient load torque is

present or when mechanical braking is used the operation takes place along the path AD2E2C.

Stopping is a special case of deceleration where the speed of a running motor is changed to zero.

Problems:

A motor having a suitable control circuit develops a torque by the relationshipTM = aω + b , where a

and b are positive constants. This motor is used to drive a load whose torque is expressed asTL = cω 2

+ d , where c and d are positive constants. The total inertia of the rotating masses is J.

a) Determine the relations amongst the constants a, b, c and d in order that the motor can

start together with the load and have an equilibrium operating speed?

b) Calculate the equilibrium operating speed?

c) Will the drive be stable at this speed?

d) Determine the initial acceleration of the drive?

e) Determine the maximum acceleration of the drive?

Solution:

a) At ω 0 , TM=b and TL=d

Hence the motor can start with the load only if b >

d TM=TL at equilibrium speed

i.e. aω + b = cω2 + d

i.e. cω2 − aω − (b − d) = 0

Hence ω = a ± a2 + 4cb − d

2c

In order that ω is finite a2 + 4cb − d> 0, which is true

+ Sign before the radical will give a positive ω as long

as a2 + 4cb − d> 0

− sign before the radial will give a positive ω only if

a > a2 + 4cb − d

2c 2c

i.e. a2 > a

2 + 4cb − d

i.e. 4cb − d< 0

i.e c<0, which is not true, since c is given to be a positive constant. Hence the + sign

before the radial only will give a positive finite equilibrium speed.

If a2 + 4cb − d >0

b) Equilibrium speed ω = a + a

2 + 4cb − d

2c

c)

dTL = 2cω and dTM = a

dω

dω

If the equilibrium speed has to be stable

dTL > dTM i.e.2cω > a

dω dω

from the answer to (b), we have

which will be always > a 2cω = a + a2 + 4cb − d

Hence, the equilibrium operating speed determined earlier is a stable point of operation

of drive.

d) Accelerating torque J dω

= TM − TL dt

Initially TM=b and TL=d

Therefore, initial acceleration =

b − d J

e) Accelerating torque J dω

= TM − TL dt

= aω − cω 2 + b − d

Therefore, acceleration A = dω

= aω

−

cω

2 +

b

−

d

dt J

This will be maximum at a speed when

dA = 0 dω

a − 2cω = 0 J

ω = a

2c Substituting this speed at which the acceleration is maximum, in the general expression for acceleration, we get

Amax = a 2

2c− a2 4c+ b − d

J

= a2 + 4cb −

d 4cJ